1. Field of the Invention
This invention relates to an automatic sphygmomanometer and, more particularly, to an automatic sphygmomanometer for measuring the systolic and diastolic blood pressures of a subject undergoing measurement by automatically identifying the appearance and disappearance of a Korotkoff sound signal.
2. Description of the Related Art
Fundamentally speaking, the measurement of blood pressure often relies upon audible recognition of Korotkoff sounds by the individual (such as a physician) taking the measurement. For this reason, even digital sphygmomanometers adapted to automatically recognize the appearance and disappearance of the Korotkoff sound signal are widely available. However, since the Korotkoff sounds are extremely low in volume, detection is easily influenced by noise arising from motion of the body and rubbing sounds produced by the cuff and tube, as a result of which systolic blood pressure and diastolic blood pressure are often perceived incorrectly. Accordingly, an expedient available in the prior art relies upon generating a pulse (heartbeat) signal in synchronization with the Korotkoff sound signal, wherein a noise gate is applied by the pulse signal to make the Korotkoff sound signal more resistant to noise. However, since this synchronized relationship is not always constant, as in cases where the pulse is irregular or the subject undergoing measurement is hypertensive, misrecognition due to a shift in synchronization often occurs. Moreover, since a pseudo-Korotkoff sound signal and pseudo-pulse signal are generated simultaneously, as when the body moves, the noise gate is rendered meaningless in terms of improving the anti-noise property.
Furthermore, the result of recognition performed by an automatic sphygmomanometer of this type and the result of recognition performed through auscultation by the person taking the measurement do not always coincide. Accordingly, it is preferred from a medical point of view that even an automatic sphygmomanometer of this kind be capable of calibration based on auscultation. In the conventional sphygmomanometer, however, the gain of an amplifier (filter amplifier) for the Korotkoff sound signal or the threshold level for recognizing the Korotkoff sound signal is fixed. Once the sphygmomanometer has been manufactured, therefore, only the quality thereof can be verified by auscultation.
Though amplifier gain, threshold level and filter constant are quantities which can be adjusted, in the prior art the signals necessary for these adjustments cannot be extracted for external use. This makes it difficult to perform the adjustments at the time of use, and confirmation by auscultation and adjustment based on sensation need to be repeated a number of times.
In addition, since only a single processing circuit for amplifying and filtering the Korotkoff sound signal is provided, it is impossible to perform separate adjustments for determining systolic blood pressure and diastolic blood pressure.